The present invention relates to a system and method for use in connection with nipped rollers and rollers nipped with shoes such as those used in papermaking, steel making, plastics calendering and printing machines, and, more particularly, to such a system and method which are capable of determining the nip width distribution between the nipped rollers.
Nipped rolls are used in a vast number of continuous process industries including papermaking, steel making, plastics calendering and printing. The characteristics of nipped rolls are particularly important in papermaking. In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine clothing or felt. Upon deposition, the white water forming a part of the stock flows through the interstices of the felt, leaving a mixture of water and fiber thereon. The felt then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the press section of the papermaking process. In the press section, two or more cooperating rolls press the fibrous web as it travels on the felt between the rolls. The rolls, in exerting a great force on the felt, cause the web traveling thereon to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several other dewatering stages.
The amount of nip pressure applied to the web is important in achieving uniform sheet characteristics. Variations in nip pressure can affect sheet moisture content and sheet properties. Excessive pressure can cause crushing of fibers as well as holes in the resulting paper product. Conventional methods addressing this problem have been inadequate and, thus, this problem persists in the press section, often resulting in paper of poor quality, having uneven surface characteristics.
Roll deflection, commonly due to sag or nip loading, is a source of uneven pressure distribution. Worn roll covers may also introduce pressure variations. Rolls have been developed which monitor and alter the roll crown to compensate for such deflection. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
Notwithstanding the problem of roll deflection, the problem of uneven loading across the roll length and in the cross machine direction persists because pressure is often unevenly applied along the roll. For example, if roll loading in a roll is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Methods have been used to discover discrepancies in applied pressure. One such method known as taking a nip impression requires stopping the roll and placing a long piece of carbon paper, foil, or impressionable film in the nip. One must load the rolls carefully to ensure that both sides, that being front and back, are loaded evenly. The pressure in the nip transfers a carbon impression, deforms the foil, or ruptures ink containing capsules in the film, indicating the width of contact. These methods for taking a nip impression are not reusable as they determine only a single event such as the highest pressure or contact width.
One of the major difficulties in using the nip impression procedure is that of evenly loading the rolls from front to back. The goal of the procedure is to measure and record the final stable loading along the length of the rolls after the initial loading. Often, during the initial loading, however, one end will contact before the other end. Thus, there are times when one end is heavily loaded while the other end is only slightly loaded. When this occurs, the nip impression shows the highly loaded condition and not the final state because the carbon paper, foils, and Prescale films record the largest width and/or highest pressures.
Another method of determining the nip pressure profile is to use a Prescale film that measures pressure. The film is fed into the nip after the rolls are loaded. Therefore, the film records the stable loaded condition rather than the greatest consequence of the loading process. Such a process eliminates the loading difficulties associated with nip impressions. Nonetheless, the Prescale films must be interpreted using a densitometer. This process is cumbersome, time consuming, and generally inefficient. Furthermore, the Prescale films are not reusable. A new piece of film must be fed into the nip after any corrective adjustments are made. Lastly, the Prescale films are temperature and moisture dependent, thus leading to inaccurate and unreliable results.
After a successful nip impression is taken, the carbon paper, foils, and Prescale films are removed from the nip and examined. Typically, the nip width is measured at twenty-one locations across the machine. These readings should be accurate to the nearest {fraction (1/64)}xe2x80x3 or 0.01xe2x80x3 for accurate interpretation. These measurements are time consuming and are subject to operator variations. Also, if the measurements require a change in the nip settings, a new piece of carbon paper, foil, or Prescale film must again be placed in the nip. A common practice is to postpone performance of a confirming test until the next available shutdown. Thus, the processing may continue at a less than optimal state.
Crown corrections are often made from nip width measurements. For simple crown corrections, the amount of correction may be estimated from:   C  =            (                        N          E          2                -                  N          C          2                    )        ⁢                            D          1                +                  D          2                            2        ⁢                  D          1                ⁢                  D          2                    
where NE is the nip width at the end of the roll,
NC is the nip width at the center of the roll, and
D1 and D2 are the roll diameters.
This equation is used throughout the paper industry for estimating crown corrections.
Various methods have been used to alleviate some of the challenges discussed previously. In U.S. Pat. No. 3,906,800 to Thettu, a reusable nip measuring device and method are disclosed. This method uses two polyamide sheets, one of which is coated with silicone rubber. When placed in the nip and when the nip is closed, the two sheets contact and fuse within the contact region. The nip is reopened, the sheets are removed, and the nip width distribution may be measured in a manner similar to carbon paper, foils, and Prescale films. Thus, the interpretation times are not improved. This method has the advantage of reusability, but is subject to the same loading path challenges in that it will record the greatest contact width and not necessarily the final stable state.
In U.S. Pat. No. 4,744,253 to Hermkens, a system is disclosed that uses ultrasonic waves to determine the thickness of a thin film sensor. The time difference between the transmission pulse and the received pulse is related to the pressure on the sensor. This time difference is used to measure the sensor cladding thickness, which is related to the applied pressure. Because this technique does not provide nip width measurements, it is inconvenient to use to make crown corrections.
In U.S. Pat. No. 4,016,756 to Kunkle, a nip load sensing device is disclosed. This device uses a bar containing load cells that are placed in the nip. The method provides discrete load reading across the machine, but does not produce nip width measurements. Thus, crown corrections do not follow directly.
U.S. Pat. No. 5,379,652 to Allonen discloses a method and device for measuring nip force and/or nip pressure in a nip. The Allonen system measures nip pressure, rather than nip width, and uses information gathered during the measurement of nip pressure to estimate nip width. Ordinarily, the piezoelectric sensors employed by Allonen require a dynamic event (such as passing through a nip) in order to operate because such sensors measure changes in pressure and vary the signal based on the amount of pressure.
U.S. Pat. No. 5,383,371 to Laitenen discloses a method and device for measurement of nip force and/or nip pressure in a nip formed by a revolving roll or a band used in the manufacture of paper. This device also employs force or pressure detectors formed of piezoelectric film. The device may be used to estimate nip width, but similarly requires a dynamic event, namely, that the measurements be made as the sensors are passed through the press nip.
U.S. Pat. No. 5,048,353 to Justus discloses a method and apparatus for roll profile measurement which also employs piezoelectric sensors.
PCT Application PCT/US96/08204 (International Publication WO 96/38718), assigned to the applicant, discloses a nip pressure and nip width sensing system, certain embodiments of which are directed to measuring nip width using pressure sensitive pads or lines.
An object of the present invention is to measure a nip width between a pair of nipped rolls.
An object of the invention is to measure nip width distribution along the length of a roll in a press nip.
Another object of the invention is to measure nip width distribution in the cross machine direction in a press nip.
Yet another object of the invention is to measure pressure distribution along the length of a roll in a press nip.
Still another object of the invention is to provide a reusable sensing system capable of measuring nip widths at multiple press nip locations, as well as pressure distribution and nip widths on different length rolls.
Moreover, it is yet another object of the invention to adjust the crown in response to irregular nip width distributions.
It is yet another object of the invention to adjust the crown in response to irregular pressure distributions.
A further object of the invention is to adjust the journal forces or applied loads in response to irregular nip width distributions.
It is yet another object of the invention to adjust the journal forces or applied loads in response to irregular pressure distributions.
Moreover, an object of the invention is to provide a method of determining the nip width in a press nip.
It is still another object of the invention to provide a method of determining the pressure in a press nip.
These and other objects of the present invention are achieved by devices for measuring a nip width between two rolls of a press nip, each of the devices including one or more sensors adapted to be placed in a stationary nip and to measure the nip width thereof, and methods for using the same. The electrical resistance of each sensor corresponds to the size of the nip width, whereby a nip width or a nip width distribution may be determined by measuring the sensor resistances. The sensors according to the invention may provide substantially direct correspondence between sensor resistance and nip width. According to methods of the present invention, nip width may be estimated from the measured resistances of force sensitive resistor sensors.
According to certain preferred embodiments, a device for measuring a nip width includes a sensor assembly. The sensor assembly includes a first strip formed of a first electrically conductive material having a resistance. The first strip has a first end and a second end and a first measuring zone between the first and second ends. A second strip is disposed adjacent the first strip and is formed of a second electrically conductive material. The second strip has a second measuring zone disposed adjacent and substantially coextensive with the first measuring zone. A gap is defined between the first and second strips and electrically isolates the first and second strips from one another. At least one of the first and second strips is deformable such that, when the device is placed in the press nip, pressure from the nip rolls forces portions of the first and second measuring zones into electrically conductive contact with one another. The area of the contact substantially directly corresponds to the nip width. The device further includes resistance measuring means for measuring an electrical resistance across the first measuring zone.
The resistance measuring means may include first and second leads. The first lead is connected to a voltage source and to the first strip adjacent the first end. The second lead is connected to the first strip adjacent the second end and to a ground. Preferably, substantially no current flows through the second strip when the first and second strips are not in contact. The first and second materials may be the same. The first material may have a higher electrical resistance than the second material. One or more compressible edge supports may be provided to maintain the gap when the sensor is unloaded and to allow the first and second strips to make contact when the sensor is placed in the press nip.
The device may include a strip and a plurality of the sensors mounted thereon. A temperature sensor may be provided to measure a temperature in the press nip and to generate a temperature compensation signal. The device may include means for detecting when an end edge of the sensor assembly is positioned within the press nip.
In a device as described above, the resistance measuring means may include a first lead connected to a ground and to the first strip adjacent the first end, a second lead connected to a ground and to the first strip adjacent the second end, a voltage source, and a third lead connected to the voltage source and to the second strip. A first resistor may be provided in the first lead between the first strip and ground, and a second resistor may be provided in the second lead between the first strip and ground.
The present invention is further directed to a device for measuring a nip width including a sensor assembly having first, second and third strips. The first strip is formed of a first electrically conductive material. The second strip is spaced apart from the first strip and is formed of a second electrically conductive material. The third strip is mounted on the second strip between the first and second strips and has first and second opposed contact surfaces. The first contact surface faces the first strip and the second contact surface electrically contacts the second strip. The third strip is formed of a semiconductor material operative to substantially prevent current flow through the semiconductor material except between the contact surfaces. A gap is defined between the first strip and the first contact surface and electrically isolates the first strip and the third strip from one another. At least one of the first strip and the second and third strips is deformable such that, when the device is placed in the press nip, the nip rolls force portions of the first strip and the first contact surface into electrically conductive contact with one another. The area of the contact substantially directly corresponds to the nip width. The device further includes resistance measuring means for measuring a resistance across the first, second and third strips. The resistance measuring means includes a voltage source, a first lead connecting the voltage source to the first strip and a second lead connecting the voltage source to the second strip.
The third strip may include segments of the semiconductor material insulated from one another. The semiconductor material may be orthotropic. According to certain preferred embodiments, the third strip has a thickness of no greater than 0.2 inch. The semiconductor material may include a filled composite comprising a non-conductive medium and conductive particles held in the non-conductive medium. The device may further include a fourth strip mounted on the first strip between the first and second strips and having first and second opposed contact surfaces, the first contact surface facing the second strip and the second contact surface electrically contacting the first strip. The fourth strip is formed of a semiconductor material operative to substantially prevent current flow through the semiconductor material except between the contact surfaces. When the device is placed in the press nip, the nip rolls force portions of the first contact surfaces of the third and fourth strips into electrically conductive contact with one another. The area of the contact substantially directly corresponds to the nip width.
The present invention is further directed to a device for measuring a nip width including a sensor assembly having first, second, and third strips and a force sensitive resistor material layer. The first strip is formed of a first electrically conductive material. The second strip is spaced apart from the first strip and is formed of a second electrically conductive material. The third strip is mounted on the second strip between the first and second strips and has first and second opposed contact surfaces. The first contact surface faces the first strip and the second contact surface electrically contacts the second strip. The third strip is formed of a semiconductor material operative to substantially prevent current flow through the semiconductor material except between the contact surfaces. The force sensitive resistive material layer is interposed between the first strip and the first contact surface. The force sensitive resistive material layer is operative to electrically isolate the first strip and the third strip from one another when in a relaxed state and to electrically connect the first strip and the third strip when a prescribed pressure is applied to the force sensitive resistive material layer. At least one of the first strip and the second and third strips are deformable such that, when the device is placed in the press nip, the nip rolls force portions of the first and second strips toward one another, thereby compressing a corresponding portion of the force sensitive resistive material layer, the area of the corresponding portion corresponding to the nip width. The device further includes resistance measuring means for measuring a resistance across the first, second and third strips. The resistance measuring means includes a voltage source, a first lead connecting the voltage source to the first strip and a second lead connecting the voltage source to the second strip.
The semiconductor layer of the sensor may be formed in the manner described above. The sensor may include a fourth strip of semiconductor material as described above.
The present invention is directed to a device for measuring a nip width between two rolls of a press nip for which a largest expected nip width is known. The device includes a sensor assembly having a sensor length and a sensor width and adapted for placement in the press nip such that the sensor length extends across the nip width and perpendicular to the roll axes. The sensor assembly includes a plurality of substantially parallel membrane switches. The membrane switches extend substantially perpendicular to the sensor length and are arranged in successive, spaced apart relation along the sensor length. At least two of the membrane switches are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. A plurality of electrically conductive lead lines are each connected to a respective one of the membrane switches.
The present invention is directed to a device for measuring a nip width between two rolls of a press nip for which a largest expected nip width is known. The device includes a sensor assembly, the sensor assembly having a sensor length and a sensor width and being adapted for placement in the press nip such that the sensor length extends across the nip width and perpendicular to the roll axes. The sensor assembly includes a first and second spaced apart lead lines extending along at least a portion of the sensor. A plurality of membrane switches are arranged in successive, spaced apart relation along the sensor length. At least two of the membrane switches are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. Each of the membrane switches is disposed between and electrically connected to each of the first and second lead lines such that each membrane switch is in electrically parallel relation to each of the other membrane switches.
A plurality of resistors may be disposed between and electrically connected to each of the first and second lead lines, each resistor being electrically connected in series relation to a respective one of the plurality of membrane switches. The membrane switches may be spaced apart along the sensor width. The first and second lead lines may extend along at least a portion of the sensor length and may be spaced apart from one another along the sensor width. Each of the membrane switches may include a dot having a diameter greater than the spacing between adjacent membrane switches along the sensor length.
The present invention is directed to a method of measuring a nip width between two rolls of a press nip. The method includes estimating a largest expected nip width between the rolls and selecting and providing a device for measuring the nip width. The device includes a plurality of sensor assemblies. Each of the sensor assemblies includes a force sensitive resistor sensor responsive to pressure applied to the force sensitive resistor to provide a variable sensor resistance as a function of the amount of pressure and the area of the pressure exerted on the sensor. Each of the sensors has a sensing length greater than the largest expected nip width. Electronics for determining the sensor resistances of the plurality of sensors are provided. The sensors are mounted in the press nip such that each of the sensors extends lengthwise across the nip width, whereby each sensor is subjected to a respective nip pressure over a contact area corresponding to the nip width at the respective sensor""s location. While the rolls are stationary, the sensor resistance of each of the sensors along the nip width is determined. A line load between the rolls is determined. Each of the sensor resistances is scaled as a function of the line load to determine a scaled line load value corresponding to each sensor resistance. A nip width corresponding to each scaled line load value is determined.
The present invention is also directed to a method of measuring a nip width between two rolls of a press nip including the steps of estimating a largest expected nip width between the rolls and selecting and providing a device for measuring the nip width as described above. Each of the sensors is characterized by determining the resistance response of each sensor as a function of pressure and contact area. The strip is mounted in the press nip such that the strip extends lengthwise along the lengths of the rolls and each of the sensors extends lengthwise across the nip width, whereby each sensor is subjected to a respective nip pressure over a contact area corresponding to the nip width at the respective location. While the rolls are stationary, the sensor resistance of each of the sensors along the nip width is determined. For each sensor, a reference line load corresponding to the determined sensor resistance is determined. A nip width corresponding to each determined reference line load is determined.
The present invention is also directed to a method of measuring a nip width between two rolls of a press nip including the steps of estimating a largest expected nip width between the rolls and selecting and providing a device for measuring the nip width as described above. Each of the sensors is characterized by determining the resistance response of each sensor as a function of nip width. The strip is mounted in the press nip such that the strip extends lengthwise along the lengths of the rolls and each of the sensors extends lengthwise across the nip width, whereby each sensor is subjected to a respective nip pressure over a contact area corresponding to the nip width at the respective location. While the rolls are stationary, the sensor resistance of each of the sensors along the nip width is determined. For each sensor, a reference nip width corresponding to the determined sensor resistance is determined.
The present invention is directed to a device for measuring a nip width between two rolls of a press nip for which a largest expected nip width is known. The device includes a sensor assembly having a sensor length and a sensor width and adapted for placement in the press nip such that the sensor length extends across the nip width and perpendicular to the roll axes. The sensor assembly includes a plurality of sensing lines arranged and configured to measure at least the largest expected nip width. The sensing lines include a force sensitive resistive material having a saturation pressure less than a prescribed nominal pressure whereby, when the device is mounted in the press nip, each sensing line positioned in the nip width will be substantially saturated. The device may include a strip and a plurality of the sensors mounted thereon. The device may include first and second opposed, flexible film layers, each of the sensing lines including a first line secured to an inner surface of the first film layer and a second line secured to an inner surface of the second film layer and facing the first line.
The device as described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend substantially perpendicular to the sensor length, the sensing lines are arranged in successive, spaced apart relation along the sensor length, and at least two of the sensing lines are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. The sensor assembly further includes a plurality of electrically conductive lead lines, each lead line connected to a respective one of the sensing lines.
The device as described above may be constructed such that the sensor assembly includes first and second spaced apart lead lines extending along at least a portion of the sensor. The sensing lines are arranged in successive, spaced apart relation along the sensor length. At least two of the sensing lines are spaced apart from one another along the sensor length a distance greater than the greatest expected nip width. Each of the sensing lines are disposed between and electrically connected to each of the first and second lead lines such that each sensing line is in electrically parallel relation to each of the other sensing lines. A plurality of resistors may be disposed between and electrically connected to each of the first and second lead lines, each resistor being electrically connected in series relation to a respective one of the plurality of sensing lines.
The device as described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend substantially parallel to the sensor length with a uniform offset spacing along the sensor length between respective ends of the sensing lines, and at least two of the ends of the sensing lines are spaced apart from one another along the sensing length a distance greater than the largest expected nip width. The sensor assembly further includes a plurality of electrically conductive lead lines, each lead line connected to a respective one of the sensing lines.
The device as described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend at an angle with respect to the sensor length with a uniform offset spacing along the sensor length between respective ends of the sensing lines, and at least two of the ends of the sensing lines being spaced apart from one another along the sensor length a distance greater than the largest expected nip width. The sensor assembly further including a plurality of electrically conductive lead lines, each the lead line connected to a respective one of the sensing lines.
The present invention is further directed to a device for measuring a nip width between two rolls of a press nip for which a largest expected nip width and a largest expected nip pressure are known. The device includes a sensor assembly a sensor length and a sensor width and adapted for placement in the press nip such that the sensor length extends across the nip width and perpendicular to the roll axes. The sensor assembly includes a plurality of sensing lines, the sensing lines arranged and configured to measure at least the largest expected nip width. The sensing lines include a force sensitive resistive material having a saturation pressure at least as great as the largest expected pressure in the nip whereby, when the device is mounted in the press nip, each sensing line positioned in the nip width will be partially actuated so that the electrical resistance of the sensing line is representative of the nip pressure on the sensing line. The device may include a strip and a plurality of the sensors mounted thereon. The device may include first and second opposed, flexible film layers, wherein each of the sensing lines including a first line secured to an inner surface of the first film layer and a second line secured to an inner surface of the second film layer and facing the first line.
The device described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend substantially perpendicular to the sensor length, the sensing lines are arranged in successive, spaced apart relation along the sensor length, and at least two of the sensing lines are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. The sensor assembly further including a plurality of electrically conductive lead lines, each lead line connected to a respective one of the sensing lines.
The device as described above may include first and second spaced apart lead lines extending along at least a portion of the sensor. The sensing lines are arranged in successive, spaced apart relation along the sensor length. At least two of the sensing lines are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. Each of the sensing lines are disposed between and electrically connected to each of the first and second lead lines such that each sensing line is in electrically parallel relation to each of the other sensing lines. A plurality of resistors may be disposed between and electrically connected to each of the first and second lead lines, each the resistor being electrically connected in series relation to a respective one of the plurality of sensing lines.
The device as described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend substantially parallel to the sensor length with a uniform offset spacing along the sensor length between respective ends of the sensing lines, and at least two of the ends of the sensing lines are spaced apart from one another along the sensing length a distance greater than the largest expected nip width. The sensor assembly further including a plurality of electrically conductive lead lines, each lead line connected to a respective one of the sensing lines.
The device described above may be constructed such that the sensing lines are substantially parallel, the sensing lines extend at an angle with respect to the sensor length with a uniform offset spacing along the sensor length between respective ends of the sensing lines, and at least two of the ends of the sensing lines are spaced apart from one another along the sensor length a distance greater than the largest expected nip width. The sensor assembly further includes a plurality of electrically conductive lead lines, each lead line connected to a respective one of the sensing lines.
According to the invention, the sensors as described above may be incorporated into a roll sensing system for measuring the pressure distribution and nip width in a nip. The sensing system comprises a strip having the sensors thereon, the strip being placed between rolls in a press nip for sensing the pressure and/or nip width at several locations along the roll. Electronics associated with the sensors communicate with an optional multiplexer and a bidirectional transmitter for signal transmission to an external signal conditioner and an external computer. The computer determines pressure values and nip width values at various locations along the strip, and communicates such values to a display which provides graphical and/or numerical data visually to the operator. Optionally, a control system can be in communication with the transmitter or the computer to initiate crown corrections in response to pressure or nip width readings.